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How information is sent from the muscles to the brain. Your muscles are pretty good, but they can't detect small differences. This experiment shows "threshold detection" — a fancy name for the point at which your muscles can detect a enough of a weight difference to say to the brain: "Hey, this one is heavier!"

A friend

A blindfold

Two large cups (16 oz. or greater)

A marker

Enough marbles, beans, or rice to fill both cups

Label one cup "A" and one cup "B."

Fill each cup exactly halfway.

Blindfold your friend.

Have your friend hold one cup in each hand. They should feel of equal weight.

Take the cups back and add a small amount of marbles, beans, or rice to cup A. ("A" for add!)

Return the cups to your friend's hands. Ask which weighs more.

If your friend says one is heavier, ask which one. Is your friend right?

If your friend says they weigh the same, take the cups back again and add a little more to cup A.

Keep taking the cups back and adding to cup A until your friend notices a difference. When he or she does, how much extra weight was needed for the muscles to send that message to the brain?